1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to an image forming apparatus using the electrophotographic photoreceptor and a method of producing electrophotographic photoreceptor.
2. Discussion of the Related Art
Image processing systems using electrophotography are remarkably developing recently. For example, laser printers and digital copiers which convert information into digital signals and record them optically are remarkably improving their print quality and reliability. There are demands for downsizing such laser printers and digital copiers and increasing the printing speed thereof as well as improving image quality. Besides, full-color laser printers and full-color digital copiers are growing in demand. Because of forming at least four-color toner images, full-color image forming apparatuses are more significantly favorable to provide higher printing speed and more compact body.
To speedup and downsize image forming apparatuses, electrophotographic photoreceptors preferably improve their durability and sensitivity much more. With regard to tandem image forming apparatuses that include four photoreceptors, it is effective to reduce the diameters of the photoreceptors for downsizing. However, a photoreceptor with a smaller diameter may be used under more sever conditions (e.g., higher speed), causing frequent replacement. Accordingly, photoreceptors to be used for high-speed and compact image forming apparatuses preferably have high sensitivity and high durability.
In general, organic photosensitive materials are widely used for electrophotographic photoreceptors because of their low cost, high manufacturability, and high environmental stability. Electrophotographic photoreceptors are broadly classified into multilayer photoreceptors in which a charge generation layer and a charge transport layer are separately provided and monolayer photoreceptors in which a single layer having functions of both generating and transporting charge is provided. Since multilayer photoreceptors are more flexible in choosing usable materials and are more improving their sensitivity, stability, and mechanical strength, they are in mainstream recently.
Various charge generation materials, such as azo pigments and phthalocyanine pigments, have been developed for use in charge generation layers of multilayer photoreceptors. Because of having high sensitivity to lights with long wavelengths of from 600 to 800 nm, phthalocyanine pigments are suitable for charge generation materials for electrophotographic printers and digital copiers containing LED or LD as a light source.
Phthalocyanine pigments include titanyl phthalocyanine pigments, metal-free phthalocyanine pigments, and hydroxygallium phthalocyanine pigments. Specific examples of titanyl phthalocyanine pigments include α-type described in Unexamined Japanese Patent Application Publication No. (hereinafter JP-A) 61-239248, Y-type described in JP-A 01-17066, I-type described in JP-A 61-109056, A-type described in JP-A 62-67094, C-type described in JP-A 63-364 and JP-A 63-366, B-type described in JP-A 2005-15682, m-type described in JP-A 63-198067, and quasi-amorphous-type described in JP-A 01-123868. Specific examples of metal-free phthalocyanine pigments include X-type described in U.S. Pat. No. 3,357,989 and 1-type described in JP-A 58-182639. Specific examples of hydroxygallium phthalocyanine pigments are described in JP-A 05-263007 and JP-A 05-279591.
Different phthalocyanine pigments have different sensitivities and stabilities. It is needless to say that a particle diameter, kinds of binder resins to be combined, a weight ratio to binder resins, kinds of charge transport materials to be combined, etc. may influence properties of phthalocyanine pigments. Further, the primary diameter of a phthalocyanine pigment may vary depending on a method of synthesizing it. Therefore, methods of synthesizing phthalocyanine pigments may influence dispersibility of the phthalocyanine pigments in charge generation layer coating liquids and electric properties of resultant photoreceptors.
JP-A 04-198367 describes a method of synthesizing a titanyl phthalocyanine pigment having an extremely small particle diameter. A charge generation material with a smaller particle diameter may improve sensitivity because the contact area with a charge transport layer increases. However, such small particles are difficult to finely disperse in a charge generation layer coating liquid, and therefore methods of dispersing them, kinds of binder resins to be combined, a ratio to binder resins may be limited. As a consequence, the charge generation material may not exert its sensitivity sufficiently.
In a charge generation layer, binder resins may significantly influence dispersion stability and crystal stability of pigments. In a case in which a ratio of pigments to binder resins is too large, the pigments may aggregate or generate crystal transition. Therefore, the ratio of pigments to binder resins is preferably varied without degrading dispersion stability of pigments.
JP-A 2007-212670 describes a technique to determine optimum binder resins for dispersing titanyl phthalocyanine pigments and an optimum ratio therebetween. More specifically, this publication describes a charge generation layer including a titanyl phthalocyanine pigment in an amount of from 50 to 350 parts by weight based on 100 parts by weight of a polyvinyl acetal resin. It is described therein that when the amount of the titanyl phthalocyanine pigment is 50 parts by weight or less, the titanyl phthalocyanine pigment does not generate sufficient amounts of charge and provides low sensitivity, and when the amount is 350 parts by weight or more, the titanyl phthalocyanine pigment is not reliably dispersed. Since there is a possibility that binder resins in a charge generation layer act as charge-trapping sites, it is generally considered that the amount of binder resins is preferably as small as possible. However, as described above, a proper amount of binder resin is used often.
It is needless to say that combinations of charge generation materials and charge transport materials influence photosensitive properties. Because of having high quantum efficiency, phthalocyanine pigments have an advantage in sensitivity and widely used as charge generation materials. On the other hand, phthalocyanine pigments generally have low ionization potential. To reduce charge injection barrier between a charge generation layer and a charge transport layer so that increase of residual potential is suppressed, a charge transport material to be used in combination with the phthalocyanine pigment preferably has an ionization potential equal to or less than that of the phthalocyanine pigment.
JP-A 2007-72139 describes such a technique in which a charge generation material having a low ionization potential reduces residual potential and improves sensitivity. However, it is generally known that charge generation materials having low ionization potentials degrade their chargeability with time.
In view of such a situation, JP 3287126 and JP-A 07-244389 each describe a charge transport layer including an antioxidant so as to make full use of charge transport materials having low ionization potentials, thereby suppressing decrease of sensitivity.
Even in a case in which phthalocyanine pigments having high sensitivity are used as charge generation materials, a photoreceptor does not provide high sensitivity if charge transportability of a charge transport layer is insufficient. JP-A 2004-002874, JP-A 11-352710, JP-A 11-143098, JP-A 10-039529, and JP-A 08-209023 each describe techniques to combine a distyryl compound which has good charge transportability as a charge transport material and a titanyl phthalocyanine pigment which has high quantum efficiency as a charge generation material. These techniques provide highly sensitive photoreceptors and reduce residual potentials thereof, however, charge stabilities thereof are poor.
To reliably provide high-grade images and high durability, provision of an undercoat layer is effective. When a photosensitive layer is provided directly on a substrate, defects present on the substrates such as scratches, impurities, and corrosions may be reflected in the resultant images, producing black dots and white spots therein. In particular, multilayer photoreceptors typically have a thin charge generation layer with a thickness of several microns or less, and therefore defects present on a substrate may cause defects on the charge generation layer as well. Besides, such a substrate has poor adhesiveness to photosensitive layers. In a case in which an undercoat layer is not provided in a photoreceptor, at the time the photoreceptor is charged, charges having an opposite polarity to those induced to a conductive substrate may locally leak and be injected to a photosensitive layer and a surface of the photoreceptor, resulting in charge reduction. As a consequence, an indefinitely large number of fine spots are developed in non-image portions of the resultant image in reversal developing methods in which non-irradiated portions on a photoreceptor correspond to non-image portions in the resultant image. This phenomenon is hereinafter referred to as background fouling. To prevent the occurrence of background fouling, provision of an undercoat layer is effective.
Undercoat layers formed with a single resin have been disclosed. For example, JP-A 47-6341 describes an undercoat layer including a cellulose nitrate, JP-A 60-66258 describes an undercoat layer including a nylon resin, JP-A 52-10138 describes an undercoat layer including a maleic acid based resin, and JP-A 58-105155 describes an undercoat layer including a polyvinyl alcohol resin.
Since such undercoat layers including a single resin have high electric resistance, residual potential may increase and image density and gradation of the resultant images may deteriorate in reversal developing methods. Moreover, because of having ion conductivity resulted from impurities, these undercoat layers may have much higher electric resistance in low-temperature and low-humidity conditions and residual potential may have large dependency on environmental conditions. In high-temperature and high-humidity conditions, these undercoat layers may have a much lower electric resistance, possibly degrading charge level. To prevent such a phenomenon, undercoat layers may be thinned as possible, however, it is difficult to optimize the thickness of the undercoat layers so that the electric resistance is stable and the occurrence of background fouling is prevented.
To solve the above-described problem, one proposed approach for controlling electric resistance of undercoat layers includes dispersing a conductive additive in an undercoat layer.
For example, JP-A 51-65942 describes an undercoat layer in which a carbon or a chalcogen substance is dispersed in a hardened resin, JP-A 52-82238 describes an undercoat layer including a thermal polymerization product formed using an isocyanate hardener in the presence of a quaternary ammonium salt, JP-A 55-130451 describes an undercoat layer including a resin in which a resistance control agent is added, and JP-A 58-93062 describes an undercoat layer including a resin in which an organic metal compound is added.
As described above, undercoat layers including a single resin have a problem of causing background fouling, and further another problem of causing interference fringes in the resultant images (this phenomenon is hereinafter referred to as moiré) when being used in an image forming apparatus using coherent light such as laser light which is generally used in reversal developing methods.
To simultaneously prevent the occurrence of moiré and control electric resistance of undercoat layers, techniques of including a pigment in an undercoat layer have been proposed.
For example, JP-A 58-58556 describes an undercoat layer in which an oxide of aluminum or tin is dispersed in a resin; JP-A 60-111255 describes an undercoat layer in which conductive particles are dispersed in a resin; JP-A 59-17557 describes an undercoat layer in which a magnetite is dispersed; JP-A 60-32054 describes an undercoat layer in which a titanium oxide and a tin oxide are dispersed in a resin; and JP-A 64-68762, JP-A 64-68763, JP-A 64-73352, JP-A 64-73353, JP-A 01-118848, and JP-A 01-118849 each describe undercoat layers in which powders of borides, nitrides, fluorides, and oxides of calcium, magnesium, and aluminum are dispersed in resins.
To sufficiently prevent the occurrence of moiré, pigments in undercoat layers preferably have a large particle diameter to some extent. However, such pigments having a large particle diameter may reduce volume ratio of the pigments in an undercoat layer, thereby increasing charge trapping sites in the undercoat layer in number.